The present invention relates to an in-band-flat-group-delay type dielectric filter having a uniform group delay time, which mainly is used in high-frequency radio equipment utilizing a high frequency band and to a linearized amplifier using the same.
Recently, many linearized amplifiers have come to be used in base station radio equipment for mobile communication systems to reduce the sizes of base stations.
FIG. 32 is a block diagram showing a feedforward amplifier as a typical example of linearized amplifiers. The feedforward amplifier shown in FIG. 32 includes delay circuits 321, directional couplers 322, 323, and 325, a main amplifier 324, an error amplifier 326, an input terminal 327, and an output terminal 328. Main signals are input from the input terminal 327 and are amplified in the main amplifier 324. In the signals amplified in the main amplifier 324, distortion occurs and only distorted components are detected in a carrier cancellation loop. The feedforward amplifier is a circuit in which only the distorted components are eliminated from the signals including the distortion, which has been amplified in the main amplifier 324, in the distortion cancellation loop, and only signals including no distortion are extracted. The details of its operation are described in xe2x80x9cHigh-Power GaAs FET Amplifiersxe2x80x9d by John L. B. Walker (issued by Artech House (Boston, London), see 7.3.2 Linearized Amplifiers). In the carrier cancellation loop and the distortion cancellation loop, in order to allow the group delay times of the two signals divided in the directional coupler 323 to coincide exactly with each other and to synthesize them in the directional coupler 325, strict and fine adjustment of the group delay times is required for the delay circuits 321.
Conventionally, in a distortion compensating circuit in a linearized amplifier, for the purpose of adjusting group delay times, a delay device using a coaxial cable such as one with a diameter of about 2 cm and a length of at least 10 m has been used in general.
However, such a delay device is large and has a great insertion loss, which have been disadvantages. The great insertion loss requires the device to have a higher output power, thus causing various problems such as an increase in the size of equipment, a high power consumption, a further complicated configuration relating to radiation, or the like, which have been obstacles to obtaining small base station equipment. Furthermore, it is required to vary the physical length of a cable for carrying out the fine adjustment of the group delay time. Therefore, each time the length is varied, it is necessary to disconnect connectors and to cut the cable, resulting in a poor working efficiency, which has been a problem.
On the other hand, a dielectric filter mainly has been used for removing undesired signals as a bandpass filter or a band stop filter, and particularly, its amplitude transfer characteristics have received attention. Therefore, conventional dielectric filters have low losses, but a deviation in group delay time depending on frequencies is great. For this reason, it has been considered that the conventional dielectric filters cannot be used for delay devices providing uniform group delays. Moreover, it has been hardly intended to flatten both amplitude characteristics and group delay frequency characteristics at the same time. In addition, there has been no example of achieving both the low loss and the reduction in size using a dielectric.
The present invention is intended to provide an in-band-flat-group-delay type dielectric filter with a small size, a low loss, and uniform-group-delay frequency characteristics.
The present invention also is intended to provide a dielectric filter in which a fine adjustment of a group delay time can be carried out easily.
Furthermore, the present invention is intended to provide a small linearized amplifier using such a dielectric filter.
An in-band-flat-group-delay type dielectric filter according to a first basic configuration of the present invention includes a plurality of dielectric coaxial resonators, a coupling circuit comprising a combination of reactive elements, with which the respective dielectric coaxial resonators are coupled to one another, and input/output terminals connected to ends of the coupling circuit. The dielectric coaxial resonators coupled to the input/output terminals have a different characteristic impedance from that of the other inter-stage dielectric coaxial resonators. According to this configuration, a small filter with a low loss and uniform-group-delay frequency characteristics can be obtained. Therefore, for example, when a cable-type delay device used in a feedforward linearized amplifier or the like is replaced by the filter with the configuration described above, due to a lower loss, a load on the amplifier is reduced and a margin in heat radiation design can be obtained, and at the same time, the size of the amplifier can be reduced. Furthermore, broad-band characteristics can be obtained and thus uniform-group-delay frequency characteristics can be obtained together with the low-loss characteristics with a small amplitude deviation. In the above-mentioned configuration, it is preferred to set the characteristic impedance of the dielectric coaxial resonators coupled to the input/output terminals to be higher than that of the other inter-stage dielectric coaxial resonators.
In the above basic configuration, it is preferable that both deviations in group delay time and in amplitude between the input/output terminals fall within predetermined certain deviation values, respectively, at the center frequency and within a specified frequency band around the center frequency at the same time, and the minimum of the group delay time within a passband is at least one nanosecond.
In the above-mentioned basic configuration, preferably, the dielectric coaxial resonators coupled to the input/output terminals are half-wave dielectric resonators with their both ends opened. According to this configuration, the Q value indicating the performance of the resonators is high, thus obtaining the effects of reducing the size and loss.
In the above-mentioned basic configuration, preferably, the dielectric coaxial resonators coupled to the input/output terminals are quarter-wave dielectric resonators with their one ends short-circuited, and the inter-stage dielectric coaxial resonators are half-wave dielectric resonators with their both ends opened. According to this configuration, a slope parameter can be varied between the input/output stages and the interstages, thus facilitating the manufacture.
In the above-mentioned basic configuration, it is possible to allow the dielectric coaxial resonators coupled to the input/output terminals to have a different characteristic impedance from that of the other inter-stage dielectric coaxial resonators by using dielectric materials with different dielectric constants. According to this configuration, the characteristic impedance can be varied easily, multistage dielectric resonators can be obtained while excellent input/output matching is maintained, the broad-band characteristics can be obtained, and low-loss characteristics with a small amplitude deviation and uniform-group-delay frequency characteristics can be obtained.
The characteristic impedance of the dielectric coaxial resonators coupled to the input/output terminals may be made different from that of the inter-stage dielectric coaxial resonators by making diameter ratios of the dielectric coaxial resonators coupled to the input/output terminals and the inter-stage dielectric coaxial resonators different. According to this configuration, the resonators are allowed to have different characteristic impedances easily. Therefore, even when, for instance, dielectric ceramic materials with the same relative dielectric constant are used, the above-mentioned configuration can be achieved, resulting in an easier manufacture.
Furthermore, it is preferable that the above-mentioned basic configuration further includes a transmission line and a directional coupler. The coupling circuit is formed of capacitors, which are formed on a coupling board formed on a dielectric substrate, for coupling the dielectric coaxial resonators. An in-band-flat-group-delay type dielectric filter, which includes the coupling board and the dielectric coaxial resonators, and the directional coupler are combined via the transmission line to form one body. According to this configuration, the loss is reduced and the size reduction also can be achieved easily.
In this configuration, it is possible to construct the coupling circuit by forming capacitors on a first dielectric substrate, forming the directional coupler on a second dielectric substrate, and then combining the first and second dielectric substrates to form one body. According to this configuration, the coupling capacitors between the stages of the resonators and the directional coupler are formed on the same dielectric substrate, thus obtaining effects of enabling a simple manufacturing process and the reductions in size and in loss.
In the above mentioned basic configuration, it is possible to regulate the resonance frequencies of the dielectric coaxial resonators by providing metallic screw tuners positioned adjacent to and in parallel to open ends of the dielectric coaxial resonators and varying the distances between the screw tuners and the dielectric coaxial resonators. According to this configuration, the regulation operation is facilitated and thus the productivity is improved drastically since the filter is a multistage filter, and in addition, an accurate regulation is possible, thus achieving a higher performance.
Furthermore, in the above-mentioned basic configuration, the resonance frequencies of the dielectric coaxial resonators can be regulated by providing metal fittings for frequency regulation electrically connected to internal conductors of the dielectric coaxial resonators and metallic screw tuners positioned adjacent to and in parallel to the metal fittings, and varying the distances between the metal fittings and the screw tuners. According to this configuration, the regulation operation is facilitated and thus the productivity is improved drastically since the filter is a multistage filter, and in addition, an accurate regulation is possible, thus achieving a higher performance.
In the above-mentioned basic configuration, metallic screw tuners provided movably in a direction perpendicular to the open ends of the respective dielectric coaxial resonators are inserted into inner holes of the dielectric coaxial resonators via dielectrics or insulators, and by varying the insertion lengths, the resonance frequencies of the dielectric coaxial resonators can be regulated. According to this configuration, the regulation operation is facilitated and thus the productivity is improved drastically since the filter is a multistage filter, and in addition, an accurate regulation is possible, thus achieving a higher performance.
In any one of the above-mentioned configurations using the screw tuners, the screw tuners may be attached to a case, and may be formed from gold, silver, or copper or may have surfaces plated with gold, silver, or copper. According to this configuration, a high no-load Q value of the resonators can be maintained, thus obtaining filter characteristics with a low loss and a high performance.
Furthermore, the frequency may be regulated by attaching the screw tuners to the case with one ends of the respective screw tuners being exposed to the outside of the case, and regulating the positions of the screw tuners from the outside of the case. According to this configuration, the regulation operation is facilitated and thus the productivity is improved drastically since the filter is a multistage filter, and in addition, an accurate regulation is possible, thus achieving a higher performance. In addition, the whole can be shielded, thus obtaining an effect of being resistant to noise jamming.
The in-band-flat-group-delay type dielectric filter of the present invention can have a configuration in which a plurality of filter blocks formed of in-band-flat-group-delay type dielectric filters with the above-mentioned basic configuration are included and the plurality of filter blocks are cascaded with a transmission line having a characteristic impedance whose value is substantially the same as that of an input/output impedance. According to this configuration, the respective filters can be regulated separately, thus highly facilitating the regulation of the whole.
In this configuration, preferably, the plurality of filter blocks are separated by shielding cases individually. According to this configuration, the characteristics of each filter block can be found accurately and therefore the regulation is facilitated.
In the above-mentioned basic configuration, it is possible that the frequency band with a uniform group delay (hereinafter referred to as a xe2x80x9cuniform-group-delay frequency bandxe2x80x9d) is within a passband in amplitude transfer characteristics and a variation in amplitude in the amplitude transfer characteristics within the uniform-group-delay frequency band is smaller than that in amplitude in the whole passband in the amplitude transfer characteristics outside the uniform-group-delay frequency band. In this configuration, it is possible that the minimum of insertion loss within the passband in the amplitude transfer characteristics falls within the uniform-group-delay frequency band. Moreover, in the above-mentioned basic configuration, it also is possible that a uniform-group-delay frequency band is within a passband in amplitude transfer characteristics and the center frequency of the uniform-group-delay frequency band is higher than that of the passband in the amplitude transfer characteristics. According to these configurations, further excellent characteristics that are desirable for a delay device can be obtained, thus obtaining a filter that can be produced and regulated easily and has a good balance between the amplitude characteristics and the delay characteristics.
In the above-mentioned basic configuration, it is possible that a uniform-group-delay frequency band is within a passband in amplitude transfer characteristics and the passband in the amplitude transfer characteristics has a width at least twice as wide as that of the uniform-group-delay frequency band. According to this configuration, the reduction in loss and uniform-group-delay frequency characteristics can be obtained and further excellent characteristics that are desirable for a delay device also can be obtained, thus obtaining a filter that can be produced and regulated easily and has a good balance between the amplitude characteristics and the delay characteristics.
In the above-mentioned basic configuration, it is possible that the frequency characteristics in group delay time have peak values at both edges of a passband in amplitude transfer characteristics and the peak value at the lower edge of the passband in the amplitude transfer characteristics is larger than that at the upper edge. It also is possible that a return loss within the uniform-group-delay frequency band has a ripple, and the minimum of the ripple within the uniform-group-delay frequency band is larger than that of ripple in a return loss outside the uniform-group-delay frequency band, and decreases from the center portion toward the both edges of the passband in the amplitude transfer characteristics. According to these configurations, further excellent characteristics that are desirable for a delay device can be obtained, thus obtaining a filter that can be produced and regulated easily and has a good balance between the amplitude characteristics and the delay characteristics.
An in-band-flat-group-delay type dielectric filter according to a second basic configuration includes a plurality of dielectric coaxial resonators, a coupling circuit comprising a combination of reactive elements, with which the respective dielectric coaxial resonators are coupled to one another, and input/output terminals connected to ends of the coupling circuit. Both deviations in group delay time and in amplitude between the input/output terminals fall within specified certain deviation values, respectively, at the same time at the center frequency and within a specified passband around the center frequency. At least one reactive element included in the coupling circuit is a variable reactive element. Thus, the group delay time within the passband can be varied.
According to this configuration, the group delay time can be varied continuously by the variable reactive element. Therefore, in a feedforward circuit in a linearized amplifier or the like, the efficiency of regulation is improved, and thus productivity and mass-productivity are improved.
The group delay time within the passband may be varied by: providing a plurality of dielectric coaxial resonators; connecting the respective adjacent dielectric coaxial resonators via at least two reactive elements connected in series; connecting a portion between the reactive elements and a ground via a variable reactive element; and varying the value of the variable reactive element.
In the above configuration, as the variable reactive element, a trimmer capacitor or a varactor diode can be used.
An in-band-flat-group-delay type dielectric filter according to a third basic configuration of the present invention includes a plurality of dielectric resonators, a main circuit comprising series coupling capacitors, with which the dielectric resonators are connected to one another, and an auxiliary circuit for coupling the main circuit with capacitors by bypass coupling. Both deviations in group delay time and in amplitude between input/output terminals fall within specified certain deviation values, respectively, at the same time at the center frequency and within a specified frequency band around the center frequency.
According to this configuration, the group delay frequency characteristics have no large peak in the vicinities of the edges of a passband and the uniform-group-delay frequency band is wide, thus achieving a number of group delays with a small number of stages.
In the above-mentioned third basic configuration, the following configuration can be obtained: two of the series coupling capacitors connect between the adjacent dielectric resonators; each one end of parallel bypass capacitors included in the auxiliary circuit is connected to a junction between the two of the series coupling capacitors; and the other ends of the adjacent parallel bypass capacitors are connected to be short circuited or via at least one of the series bypass capacitors.
In the third basic configuration, the following configuration also can be obtained: one of the series coupling capacitors connects between the adjacent dielectric resonators; each one end of parallel bypass capacitors included in the auxiliary circuit is connected to a junction between the series coupling capacitors; and the other ends of the adjacent parallel bypass capacitors are connected to be short circuited or via at least one of the series bypass capacitors.
In the above configuration, at least one of the parallel bypass capacitors may be opened. In addition, at least one of the series bypass capacitors may be short circuited.
In any one of the configurations according to the third basic configuration described above, the following configuration can be obtained. That is, the frequency characteristics in group delay have a peak value at the lower edge of a passband in amplitude transfer characteristics, and uniform-group-delay frequency characteristics within the passband. In a higher frequency band than the upper edge of the passband, the frequency characteristics in group delay frequency characteristics do not increase from a uniform group delay time within the passband but decrease.
A linearized amplifier of the present invention includes a dielectric filter with any one of the above-mentioned configurations, and a group delay time in a distortion compensating circuit is regulated by the dielectric filter. This configuration achieves the reductions in size of base station radio equipment and in power consumption, the simplification of configuration relating to radiation, and the like.
In the linearized amplifier with this configuration, the distortion compensating circuit can be designed as a feedforward type. According to this configuration, the in-band-flat-group-delay type dielectric filter is inserted into the main path in which a large current passes, thus further improving the effects of the reductions in size of base station radio equipment and in power consumption, the simplification of configuration relating to radiation, and the like.
In the linearized amplifier with the above-mentioned configuration, it is possible to set the uniform-group-delay frequency band width in the dielectric filter to be at least three times as wide as a required bandwidth of the amplifier. According to this configuration, the intermodulation distortion of third order or higher in the amplifier can be compensated, thus obtaining an amplifier causing a low distortion.